Traditionally, sound has been recorded in consumer applications, such as dictation recorders and telephone answering machines, by way of analog recording. In analog recording, an electrical representation of sound waves is preserved on a storage medium in such a way that the representation can be read to reproduce the original sound waves. Analog recording technology is old and relatively straightforward. In one common type of recording, a sound wave picked up and converted into an electrical signal by a microphone, is amplified and recorded onto magnetic tape passing under a transducer simply by passing the amplified electrical signal to the transducer. To reproduce the sound, the reverse is performed. The magnetic tape passing under the transducer creates an electrical signal in the transducer, which upon amplification can be played via a speaker to reproduce the recorded sound.
This approach, while simple, has several disadvantages. Analog recording units employing magnetic tape have a number of moving parts, are mechanically complicated, and typically consume a substantial amount of power. Moreover, because magnetic tape is a long, linear medium, random access to any location on the tape may require spooling through a substantial length of tape. Finally, it is difficult, expensive, and time-consuming to be able to insert and delete passages between recorded segments of tape.
Accordingly, digital audio recording is now replacing analog audio recording in many different applications. In digital recording, the sound wave picked up and converted into an electrical signal by a microphone is processed by a computer and turned into a sequence of numeric codes, which are then stored in digital memory. Later, these numeric codes can be processed by the computer back into an electrical signal for reproduction via a speaker.
Digital recordings have several advantages over their analog counterparts. First, by employing digital memory for storage, there is no need for any moving parts. Accordingly, digital audio recording units can be more reliable and durable than comparable analog recorders. Also, because no motors or magnetic transducers are necessary, digital recorders can consume less power. Finally, because recording is not accomplished on a long piece of linear medium, such as a magnetic tape, it is possible for digital memory to be accessed in a non-sequential manner without the delay inherent in spooling a magnetic tape to a specified location.
Several factors are making the adoption of digital audio recording more feasible for consumer applications. First, the cost of digital components, particularly memory, has decreased drastically within the last several years. Second, only recently have digital signal processors with sufficient throughput and capable of performing audio compression been developed.
Even with advanced compression techniques, providing adequate recording quality still requires a large amount of digital storage. As much as prices have come down, the cost of digital storage remains a limiting factor. For example, to record five minutes of a single channel of CD-quality sound (sampled at 44.1 kHz with 16-bit sample resolution), without compression, over 200 megabits of storage are necessary. However, this storage requirement can be reduced in several ways. The sample rate can be reduced; this will decrease the frequency response of the recorded sound, causing high frequencies to be lost. The sample resolution can be reduced; this increases the signal-to-noise ratio in the sound. Compression schemes can be used; these typically affect reproduction fidelity in various ways. By employing all of these techniques in combination, the most advanced compression schemes permit five minutes of audio to be reduced to less than two megabits of storage, less than one percent of the original storage requirement. This level of compression will provide reproduction quality acceptable for voice recording, for example in telephone answering machines. However, two or more megabits of memory is still a significant amount of memory for a consumer-oriented device, intended to be sold at a relatively low price.
Traditional low-cost digital memory devices have a significant disadvantage. When power is removed, the memory contents are permanently lost. For example, the most common and least expensive type of digital memory usable for audio recording is dynamic random-access memory (dynamic RAM). Audio grade dynamic RAM, which may be partially defective (and thus not usable in data-storage applications) is known as ARAM. When power is disconnected from ARAM, the memory contents are lost. Moreover, ARAM must be periodically "refreshed" by electrically stimulating the memory cells. For these reasons, a battery backup must be provided to preserve ARAM contents when the device is removed from its primary power source. This is inconvenient for the user and adds bulk and expense to a device that uses ARAM. Moreover, additional circuitry can be necessary to provide the necessary refresh signals to the ARAM.
Despite their disadvantages, ARAM devices are in relatively high demand because of their low price point. Accordingly, ARAM devices are sometimes in short supply, causing their price advantage to be nullified.
Another type of volatile digital memory is known as static RAM. Static RAM is typically very fast, but is also power-consuming and expensive. No refresh signals are necessary, but like dynamic RAM, power must be continually supplied to the device, or memory contents will be permanently lost.
Several types of non-volatile memory are also available. EEPROM, or Electrically Erasable Programmable Read-Only Memory, is expensive in the quantities and densities necessary for audio storage. So-called bubble memory is also available; it, too, is expensive, and is generally too slow for advantageous use in audio recording. Finally, flash memory is available. Traditionally, flash memory has been expensive, and very slow to erase and to write. In recent years, the time required to program flash memory has been reduced, and it is now usable for audio recording. However, most varieties are still very expensive. Moreover, flash memory is subject to a burnout effect. After a limited number of re-writes, the device will wear out and become unusable.
In light of the disadvantages of the various volatile and non-volatile digital storage options for voice recording, there is a recognized need for a digital audio recording system that is inexpensive and reliable and uses sufficiently fast non-volatile memory.